Help me decide who of those units are better:

Soleil is probably your best option if you want to have a strong and self-reliant infantry sword. She has excellent stats all around and has an obvious focus on offense, which surprisingly doesn't exclude her from goin for a more enemy phase build.

Corrin has a unique upgrade but he is not really a core unit on himself and functions mostly as a support even though he has good stats all around.

With that said, you probably can't go wrong with either of them. I would personally go for Soleil, due to how flexible she is when it comes to her build, which is a huge advantage over Corrin who can't really work without Yato.
In the end it depends on whether you want that sword unit to be the main carry of your team or whether you want it to just finalize/fill the missing piece.

Also, Marth and Fir are not neccesarily outclassed by these two. Marth also has good stats (+ you should never underestimate the power of renewal bulit-in and dragon murdering effect) and serves as a better general support than Corrin, whereas Fir is arguably slightly weaker than Soleil when it comes to flexibility and general skill build, but refined nameless blade combining 16mt and slaying + wo dao effect is extremely powerful.

Another honorable mention goes to Eirika, who is really fun to play with due to all the buff interactions and it's satisfying to see it all work correctly when you pull off the max buffs.

I wouldn't say Corrin outclasses Marth, as Marth's drive refine affects all allies whereas Corrin only affects the support Ally. They are both good in very different ways. I myself only have ever pulled 1 Marth, but the Drive Falchion with Sol (might switch it up to Aether) and Renewal means he's regaining HP like crazy.

Soleil fits into way more teams and is much stronger. Slaying Edge + Wrath + Speed seal is worth a look for her to secure more doubles in both phases.

Corrin is all about supporting your favorite unit. Support a 5*+10 F Kana to a 50/57/58/48/41 statline.

Soleil's like a percent or two off from being as good as the top tier swords.... Male Corrin is a cheerleader who you might as well 4*+10 (merge 5 star with refine back down) for the same results but way less investment.

I would say Soleil who has greater offensive stat's.
But a question, how do you have iv's on unit summary screen ?

I disagree that Marth is outclassed, I'd say he's better than Corrin. +2 x 3 allies is more than +4 x 1 ally, and you can use him in any team instead of being stuck with his support. In terms of stats they're very similar. Double Renewal + Reciprocal Aid means Marth can heal his allies too. And as another bonus he's effective against dragons.

If you're going to use them in Arena, I'd recommend Soliel. She scores higher because she has higher stat total than the other three, and Marth/Corrin's lower atk can lead to problems killing all those bulky armours. She doesn't help her team but she's better in a straight fight

For non-Arena stuff, I'd prefer Marth. He may be a bit worse in combat but he has so much utility to make up for it.